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Temperatures in the upper 200 km of the mantle are ~200 K higher than assumed
in canonical geotherms*Don L. Anderson
Because of…1. Anharmonicity, anisotropy, anelasticity
2. Non-linear conductivity (insulation)3. Thick boundary layer (seismology)
4. Secular cooling (Lord Kelvin)5. Radioactivity (Rutherford)
6. Seismic properties
*mantle potential temperatures at ~200 km depth are higher than at ~2800 km depth
Temperatures in hypothetical deep ‘Plume Generation Zones’ (PGEs)are >300 C colder than in the surface boundary layer
DEPTH
McKenzie & Bickle* ignore U,Th,K; therefore, their ‘ambient’ mantle is colder than in more realistic models.
*Cambridge geophysicists have now abandoned the assumptions behind their geotherm but geochemists still use it to define excess T.
PGE
D”
Depth (km)
Schuberth et al.
The upper boundary layer is hotter/thicker & the lower boundary layer is colder than assumed in Canonical
Geotherms such as McKenzie & Bickle (1988)
Internally heated & thermodynamically self-consistent geotherm derived from
fluid dynamics
The recognition that mantle potential temperatures at ~200 km
depth are higher than between ~ 400-2800 km depth is the most significant
& far-reaching development in mantle petrology & geochemistry since Birch &
Bullen established the non-adiabaticity of the mantle (superadiabatic thermal gradient
above 200 km, subadiabatic gradient below) .
Tdepth
High Tp in the shallow mantle is consistent with petrology (Hirschmann, Presnell)[the BL is mainly buoyant refractory harzburgite, not fertile pyrolite]
Geophysically inferred midplate & back-arc mantle temperatures are typically ~1600 C at ~200 km depth, with 1-2 % melt content*
M. Tumanian et al. / Earth-Science Reviews 114 (2012)
*this is just one example of the over-whelming geophysical evidence for Tp>1500 C in the surface boundary layer (Region B)
A back-arc thermal environment
1600 C
PLATE
Low-velocity zone
Intra-plate magmas such as Hawaiian tholeiites are derived from the low-velocity zone (LVZ) part of the sheared surface boundary layer (LLAMA). They are shear-driven not buoyancy driven.
The upper 220 km of the mantle (REGION B) is a thermal, shear & lithologic boundary layer & the source of midplate magmas.
200 km
FOZO
1600 C
MORB
MORB
LVZ
LITHOSPHERE
Ocean Island
220 kmOIB
UPDATE OF CLASSICAL PHYSICS-BASED PLATE MODELS (Birch, Elsasser, Uyeda, Hager…)*
after Hirschmann
*not Morgan, Schilling, Hart, DePaolo, Campbell…
-200 C -200 C
INSULATING LID
See also Doglioni et al., On the shallow origin of hotspots…: GSA Sp. Paper 388, 735-749, 2005.
Canonical 1600 K adiabat
Geotherm derived from seismic gradients
CONDUCTION REGIONSUBADIABATIC REGION
Thermal bump region (OIB source)
It has long been known that seismic gradients imply subadiabaticity over most of the mantle (Bullen, Birch)
Xu
T
Depth
Boundary layer
Midplate
Ridge adiabat
LLAMA(shearing)
Plate (conducting)
Depth
16001400
T oC
T
Depth
B
D”
TZ
CMB
Geotherms illustrating the thermal bump and subadiabaticity
UPPER MANTLE
LOWER MANTLE
The highest potential temperature in the mantle is near 200 km. Tectonic processes (shear, delamination) are required to access this.
ridge midplate
bump
(& backarc)
400200
LVZ
MID-PLATE BOUNDARY LAYER VOLCANOES
Leahy et al.
Kawakatsu et al
“hotspot” & back-arc magmas are extracted from the thermal bump region of the surface boundary layer
Common Components (FOZO)
1600 C
AMBIENT MIDPLATE MANTLE TEMPERATURES REACH 1600 C
The upper boundary layer (BL) of the mantle is hotter than assumed in geochemistry; the deeper ‘depleted mantle’ (DM) source of MORB is ~200 K colder than ambient shallow (subplate) mantle*.
Hawaiian magmas are from ambient BL mantle; no localized or ‘excess’
temperature is required.
*all terrestrial ‘intra-plate hotspot’ magmas are derived from the surface boundary layer. MORB & near-
ridge ‘hotspots’ are from the cooler TZ.
Norman SleepJason Phipps Morgan
Ridge
MORB
anisotropic
Sub-Adiabatic3D Passive Upwellings
Lateral plumes
Standard Model
Long-Distance Lateral flow of plume material…avoiding thin spots (ridges)
Ridge source
hot
“ambient”
hot
Ridge source
LLAMA Boundary (thermal bump) Layer (thick plate)Model+200 C
-200 C
See “shallow origin of hotspots…”, C. Doglioni
Gives an oceanic plateau when a triple junction migrates overhead
O
CMB
Thermal max in upper mantle exists without “plume-fed asthenosphere” or core heat
Melts can exist in the BL
Effects of secular cooling, radioactivity, thermodynamics (& sphericity)
Subadiabatic gradient (Jeanloz, Morris, Schuberth)
“… most geochemists & geophysicists have taken the adiabatic concept dogmatically... Such a view impact(s)… petrology, geochemistry & mineral physics.” Matyska&Yuen(2002)
OIB
MORB
A
B’
B”
C’
C’’
D’
D”
Crust
LID
220-
410
650
Lower
Mantle
Tp
BL
BL
LVLG
L
Region B Moho-220 km
Region D”
Subadiabatic geotherm
Deep Tp is colder than B
slabsTZ
OIB &Back-arc magmas
MORB
No infinite energy source; no 2nd Law violationsDecaying T boundary condition
Anderson, J.Petr. 2011
Maggi et al.
Some ridge segments are underlain by “feeders” that can be traced to >400 km depth, particularly with anisotropic
tomography (upwelling fabric)
Ridges cannot represent ambient midplate or back-arc mantle
THE QUESTION NOW IS, WHERE DOES MORB COME FROM? RIDGES HAVE DEEP FEEDERS
6:1 vertical exaggeration
Only ridge-related swells have such deep roots
Passive upwellings are broad & sluggish, to compensate for narrow fast downwellings
Ridge crests occur above ~2000 km broad 3D passive upwellings…’hotspots’ are secondary or satellite shear-driven upwellings
1000-2000 km
Near-ridge ‘hotspots’ sample deep & are coolish compared to midplate volcanoes
MORB
OIB
Along-ridge profile
Ridge-normal profile ridge
R i d g e
geotherms
Ridge adiabat
T
TZ
TZ
OIB
RIDGE FEEDERS
True intra-plate hotspots do not have deep feeders
*Laminated Lithologies & Aligned Melt Accumulations (Anderson, J. Petr. 2011)
LLAMA* Shear Boundary Layer Model
Lateral variation in relative delay times are due to plate & LVZ structure & subplate anisotropy, not to deep mantle plumes
teleseismic rays
west
underplate
SKS very lateS early S late
HOT FRACTURE ZONES & ROOTS OF SWELLS PERTURB MANTLE FLOW
Mantle potential temperatures at ~200 km depth are higher than between ~ 400-2800 km depth. This is the most significant & far-
reaching development in mantle petrology & geochemistry since Birch & Bullen established
the non-adiabaticity (subadiabatic thermal gradient) of the mantle from seismology &
physics 60 years ago. High temperatures can only be accessed where laminar flow is disturbed (delamination, FZs, convergence).
TAKE-AWAY MESSAGE
200 Myr of oceanic crust accumulation
TRANSITION ZONE (TZ)
REGION BSuper-adiabatic boundary layer
Thermal max
600 km
300 km
Tp decreases with depth
600 km
Thus, the ‘new’* Paradigm
(RIP)
(* actually due to Birch, Tatsumoto, J.Tuzo Wilson)
Shear strain
“fixed”Hawaii source
MORB source
Shear-driven magma segregation
Mesosphere (TZ)
LIDLVZ LLAMA
200
400
Ridges are fed by broad 3D upwellings plus lateral flow along & toward ridges
Intraplate orogenic magmas (Deccan, Karoo, Siberia) are shear-driven from the 200 km thick shear BL (LLAMA)
ridge
kmCold slabs
SUMMARY
Net W-ward drift is an additional source of shear (no plate is stationary)
LithosphereLidLow-wavespeed Anisotropic &Melt-accumulation zones
ASTHENOSPHERE
Viscosity
Temperature
The active layer
Interesting region for seismology but unimportant for geochemistry
LLAMA
Physics-based models (e.g. Birch) are paradox-free because the heatflow,
helium, neon, Pb, Th, TiTaNb, FOZO, DNb, OIB, chondritic, mass balance, excess
temperature, ambient mantle, subsidence, LAB…paradoxes & the
Common Component Conundrum are all artificial results of unphysical &
unnecessary assumptions in the canonical models of geochemistry & petrology.
SKIP
The questions are no longer “From what depth are plumes emitted?” and “Are Hawaiian magmas hotter than MORB & ambient mantle?”, but rather “With a 200 km thick insulating boundary layer are plumes needed at all?”
“Considering the subadiabatic nature of the deep mantle geotherm (in the presence of internal heating & cold slabs) are plumes even useful for the purpose intended?”
“If the boundary layer is shear-, rather than buoyancy-driven, do we need the plume concept?”
Magmas are delivered to the Earth’s surface not by active buoyancy-driven upwellings but by shear-induced magma segregation (Kohlsteadt, Holtzman, Doglioni, Conrad), magmafracture and passive upwellings. “Active” upwellings (plumes, jets) play little role in an isolated planet with no external sources of energy and material. This is a simple consequence of the 2nd Law of thermodynamics (Lord Kelvin)…secular cooling also implies subadiabaticity in an isolated cooling planet.
Midplate mantle
Passive upwelling mantle (no surface boundary layer)
Magma potential temperatures depend on age of plate and depth of extraction (modified from Herzberg).
Inferred T & P of midplate magmas are all in the boundary layer, which has to hotter than at mature spreading ridges
PETROLOGICALLY INFERRED TEMPERATURES IN THE MANTLE(Herzberg, annotated) Typical BL
temperatures inferred from seismology & mineral physics
Mantle under large plates cannot be as cold as at mature ridges
upwellings
Ridges are fed by broad passive upwellings from as deep as the transition zone (TZ). They are not active thermal plumes & are mainly apparent in anisotropic tomography.
(Lubimova, MacDonald, Ness)
U, Th, K and other LIL are concentrated in the crust & the upper mantle boundary layer during the radial zone refining associated with accretion (Birch, Tatsumoto…). This accentuates the thermal bump.
Francis Birch (1952 & his 1965 GSA Presidential Address)... The Earth started hot & differentiated, & put most of its radioactive elements toward the top…which becomes hot.This is ignored in all standard petrology & geochemical models.
“The transition region is the key to a variety of geophysical problems…”
…including the source of mid-ocean ridge basalts.
IN
OUT
OUT
Doglioni et al. 2007 ESR
Plate motions plus net westward drift of the lid-lithosphere-plate system (LLAMA) create anisotropy & cause shear-driven melt segregation in the upper ~200-km of the mantle, a shear boundary layer
Westward drift of the outer boundary layer of the mantle also shows up as a toroidal component in plate motions (which is
added to plate motions in the no-net-rotation frame)
Thermal bump
Earth-like parameters (U,Th,K)
Geotherms derived from fluid- & thermo-dynamics
Region D”
Region B
(*Jeanloz, Moore, Jarvis, Tackley, Stevenson, Butler, Sinha, Schuberth, Bunge, Lowman etc.)
With realistic parameters most of the mantle in fluid dynamic models is subadiabatic *, in agreement with classical seismology
[low Rayleigh numbers, Ra, are appropriate for chemically stratified mantle (Birch)]
No U,Th,K
Unfortunately, many geochemists still assume adiabaticity & maximum upper mantle temperatures of ~1300 C
r
What is geophysically unique about the mantle around hotspots?
Anisotropy (not local heatflow, temperature or low wave speed)
A partially molten sheared thermal
boundary layer
(LLAMA)
laminated
ridge
BL
NETTLES AND DZIEWONSKI
wavespeed
anisotropy
Hawaii
LLAMA
1600 C
~1300 C
Max melt
shear